Low friction O-ring for use in a carbon face seal

ABSTRACT

A stator seal assembly is provided that includes a seal case, an O-ring and a stator ring. The seal case has an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween. The O-ring is coupled to the seal case inner peripheral wall and disposed in the annular cavity, and is constructed of a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50. The stator ring is disposed within the seal case annular cavity.

TECHNICAL FIELD

The present invention relates to gas turbine engines and, more particularly, to a seal assembly for use in gas turbine engines, gas turbine engine starters, and auxiliary power units.

BACKGROUND

Air turbine starters (ATS) may be used to initiate rotation of many relatively large turbine engines, including turbofan jet engines. Typically, the ATS is disposed within a housing and includes a turbine section and an output section. The turbine section includes a turbine wheel coupled to an output shaft. When high pressure fluid, such as compressed air, enters the ATS, it contacts the turbine wheel and causes it to rotate at a relatively high rate of speed. As the turbine wheel rotates, the output shaft also rotates, which in turn, causes the jet engine to rotate. The ATS output shaft may be rotationally mounted within the housing using one or more bearing assemblies. Preferably, a lubricant, such as oil, is contained within the housing to lubricate the output shaft and bearing assemblies.

In order to prevent the lubricant from leaking out of the output section into the turbine section, a seal assembly is typically provided between the two sections. The seal assembly may be a face seal that includes a rotor, a seal ring, and a seal case. The rotor is mounted on the turbine wheel output shaft and has an axially facing flange that extends radially away from the shaft. The seal case may have a neck and may be mounted to the ATS housing in the turbine section and surrounds the turbine wheel output shaft, while the seal ring is housed within the seal case. The seal ring has a spring pre-load that supplies force thereto to sealingly engage the seal ring with the rotor axially facing flange. An O-ring is typically mounted on the seal case in the space within which the seal ring is housed and is used to maintain conformal radial contact between the inner diameter of the seal ring and the outer diameter of the seal case neck. Thus, the O-ring acts as a secondary seal by deforming out of its original round shape at the zones of contact with the seal ring and seal case.

Typically, O-rings are made of an elastomer (rubber) and may vary in hardness. Material hardness may be measured by a durometer instrument, which may include a contact tip for pressing into the material. The resistance of the material toward indentation motion of the tip is then monitored. Durometer measurements are communicated using various Shore hardness scales, including, but not limited to Shore A, Shore D, Shore H, and Shore M scales. In many cases, Shore M scales are used to indicate the hardness of plastics. For instance, a material having a Shore M hardness value of below 55 is made of a relatively soft material, while a Shore M hardness value of above 80 is a relatively hard material.

In the past, O-rings having a Shore M hardness of between 55 and 80 have been used in conjunction with face seal assemblies. However, it has been found that during operation, some of these O-rings may become deformed or compression set due to prolonged exposure to heat. When this occurs, the O-ring may lose its original round shape if in its free state and may form permanent flat sections at the seal case and seal ring contact zones. As a result, if rotor axial movement occurs, the O-ring may supply an excessive amount of friction against the return motion of the seal ring. High axial O-ring friction may prevent the seal ring from properly sealing against the rotor. Consequently, lubricant may leak out of the ATS gearbox at the turbine section and/or output section and assembly maintenance and/or repair may need to be more frequently performed.

Identifying suitable elastomers that may be less likely to demonstrate excessive axial friction when compression set within the seal assembly has been somewhat difficult. In particular, two materials may have the same hardness durometer measurements, but one material may be better-suited for use in the seal assembly than the other material. Because there may be numerous materials that may have similar durometer values, identification of appropriate O-ring materials is currently performed by trial and error. This method of identification may be extremely time-consuming and does not take into consideration whether the elastomer properties may change over time and after exposure to friction.

Accordingly, it is desirable to a seal assembly that is less time-consuming to design and manufacture. It is also desirable for the seal assembly to supply an appropriate amount of friction against a seal ring to provent the seal ring from becoming dislocated from the seal rotor in the event of seal rotor axial movement. In addition, it is desirable to have a seal assembly that minimizes leakage at the turbine section and/or output section of the ATS. Furthermore, other desirable features and characteristics of the present invention will become apparent from the subsequent detailed description of the invention and the appended claims, taken in conjunction with the accompanying drawings and this background of the invention.

BRIEF SUMMARY

The present invention provides a stator seal assembly including a seal case, an O-ring and a stator ring. The seal case has an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween. The O-ring is coupled to the seal case inner peripheral wall and disposed in the annular cavity, and is constructed of a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50. The stator ring is disposed within the seal case annular cavity.

In one embodiment, and by way of example only, a seal assembly is provided that includes a seal case, an O-ring, and a stator ring. The seal case has an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween. The O-ring is coupled to the seal case inner peripheral wall and disposed in the annular cavity and comprises a material having stress versus strain ratio of less than about 5.0 ksi and a Shore M value of about 50. The stator ring is disposed within the seal case annular cavity.

In another embodiment, and by way of example only, a face seal assembly is provided that includes a seal rotor, a seal case, an O-ring, and a stator ring. The seal rotor is adapted to mount to a shaft and has an axially facing flange. The seal case has an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween. The O-ring is coupled to the seal case inner peripheral wall and disposed in the annular cavity and comprises a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50. The stator ring is disposed within the seal case annular cavity and is configured to engage the axially facing flange.

In still another embodiment, and by way of example only, an air turbine starter is provided that includes a rotatable shaft and a seal assembly. The seal assembly is coupled to the rotatable shaft and includes a seal rotor, a seal case, an O-ring, and a seal ring. The seal rotor is coupled to the rotatable shaft and has an axially facing flange. The seal case has an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween. The O-ring is coupled to the seal case inner peripheral wall and disposed in the annular cavity, and comprises a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50. The stator ring is disposed within the seal case annular cavity and is configured to engage the axially facing flange

Other independent features and advantages of the preferred seal assembly will become apparent from the following detailed description, taken in conjunction with the accompanying drawings which illustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross section view of an air turbine starter that may use the seal assembly;

FIG. 2 is a cross section view exemplary stator seal assembly that may be implemented into the air turbine starter of FIG. 1; and

FIG. 3 is a cross section view of an exemplary seal case that may be implemented into the stator seal assembly of FIG. 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

The following detailed description of the invention is merely exemplary in nature and is not intended to limit the invention or the application and uses of the invention. Furthermore, there is no intention to be bound by any theory presented in the preceding background of the invention or the following detailed description of the invention. Before proceeding with the detailed description, it should be appreciated that the present invention is not limited to use in conjunction with a specific type of rotating machine. Thus, although the present invention is, for convenience of explanation, depicted and described as being implemented in an air turbine starter, it should be appreciated that it can be implemented in numerous other machines including, but not limited to, a solenoid, a control valve, a solenoid control valve, a hydraulic pump mechanical face seal, a water pump mechanical face seal, a petrochemical mechanical face seal, a pressure accumulator piston, a pressure transducer, or any other suitable machine or application.

Turning now to the description, a cross section view of an exemplary air turbine starter (ATS) that is used to initiate the rotation of a larger turbine, such as a turbofan jet engine, is depicted in FIG. 1. The ATS 100 is enclosed within a housing assembly 102 that includes at least a turbine section 104 and an output section 106. The housing assembly 102 may be made up of two or more parts that are combined together or may be integrally formed as a single piece. The turbine section 104 receives compressed air and directs the compressed air to various sections of the ATS 100. The turbine section 104 includes a turbine wheel 108 that is rotationally mounted therein. The turbine wheel 108 has an output shaft 110 that extends from a hub 126, through the housing assembly 102, and into the housing assembly output section 106. The turbine wheel output shaft 110 is rotationally mounted in the turbine section 104 by bearing assemblies 114. The turbine output shaft 110 is also coupled to a drive shaft 116 that is coupled to a turbine output shaft 118. The output shaft 118 is, in turn, coupled to the turbofan jet engine gearbox (not illustrated).

FIG. 2 illustrates a close up view of a section of the ATS 100 within which a face seal assembly 130 is disposed. The face seal assembly 130 provides a fluid tight seal between the rotating turbine wheel 108 and the oil held inside of the turbine section 104 and the housing assembly output section 106. Though not explicitly depicted, it should be appreciated that although one face seal assembly 130 is depicted in FIG. 2, another face seal assembly 130 may also be included in the ATS 100 that seals the output shaft 118. The face seal assembly 130 includes a rotor 132 and a stator seal assembly 134. The rotor 132 is mounted on the turbine wheel output shaft 110, and has an axially facing flange 136 that extends radially outwardly away from the turbine wheel output shaft 110. The stator seal assembly 134 includes a seal case 138, a seal stator ring 140, a spring washer 142, a retaining ring 144, and an O-ring 146. These components are shown in further detail in FIG. 3.

With reference to FIG. 3, the seal case 138 is mounted to the housing 102 and surrounds the turbine wheel output shaft 110. The seal case 138 is generally annular in shape and, in the depicted embodiment, is formed from two substantially annular hubs, a first hub 148 and a second hub 150. The first hub 148 surrounds the second hub 150, and includes an inner surface 152, an outer surface 154, a first end 156, and a second end 158. Similarly, the second hub 150 includes an inner surface 160, an outer surface 162, a first end 164, and a second end 166. The seal stator ring 140 is housed within the seal case 138 and, as previously mentioned, sealingly engages the axially facing flange 136 of the rotor 132. A substantially flat radial end wall 168 couples the first hub second end 156 to the second hub second end 166, forming an annulus 170 between the second hub outer surface 162 and a portion of the first hub inner surface 152. As depicted in FIG. 2, the spring washer 142 is inserted in the annulus 170 and exerts an axial force on the seal stator ring 140. A groove 172 is formed in the first hub inner surface 152 that is configured to receive the retaining ring 144. Specifically, the retaining ring 144 is inserted into the groove 172 and holds the seal stator ring 140 in place within the seal case 138 against the axial force of the spring washer 144.

The O-ring 146 is mounted on the second hub outer surface 162. Preferably, the O-ring 146 is toroidally-shaped and constructed of material capable of providing a sufficiently low amount of friction when used as a radial compression O-ring between the seal stator ring 140 and seal case 138 such that the spring preload of the spring washer 142 may overcome the O-ring friction when the seal stator ring 140 moves axially. In one exemplary embodiment, suitable materials have a Shore M value of between about 30 and 50, but also a suitable stress versus strain ratio (“compressive modulus”). The compressive modulus of a material may be measured in any one of numerous manners. In one example, the compressive modulus is measured by determining the force that is needed to compress a complete surface of a component, such as a rubber component in contact with the seal case 138 and the seal ring 140. Preferably, materials appropriate for constructing the O-ring 146 have a compressive modulus of less than about 5.0 ksi, more preferably between about 5.0 and 0.1 ksi, and most preferably about 1.4 ksi.

In other exemplary embodiments, other material specifications may be dependent on the particular application in which the O-ring 146 will be implemented. For example, the O-ring 146 in ATS 100 is preferably resistant to structural degradation due to exposure to oils. Thus, the O-ring is preferably constructed of a fluorocarbon, including, but not limited to perfluorocarbon, peroxide cured fluorocarbon, GLT fluorocarbon (available through DuPont Dow Elastomers of Wilmington, Del.), and GFLT fluorocarbon (available through DuPont Dow Elastomers of Wilmington, Del.). The O-ring elastomer could also be any of the available elastomers or products including but not limited to ethylene propylene, nitrile, butadiene, chloroprene, butyl, isoprene, silicone, fluorosilicone, other fluoroelastomers, such as Sifel™ (available through Shin-Etsu Silicones of America, Inc. of Akron, Ohio), Kalrez® (available through DuPont Dow Elastomers of Wilmington, Del.), Viton® (available through DuPont Dow Elastomers of Wilmington, Del.), Chemraz® (available through Greene Tweed of Kulpsville, Pa.), Fluorel (available through 3M Corporation of Minnesota), Omniflex™ (available through Saint-Gobain Performance Plastics Corporation of Garden Grove, CA), Aflas® (available through Asahi Glass Co. of Tokyo, Japan), polyurethane, polyester, or, in accordance with ASTM D1418 abbreviations, the elastomer compounds of ACM, AEM, CSM, EPDM, EPM, FKM, FEPM, FFKM, CO, ECO, CO, ECO, BR, CR, IIR, CIIR, IR, NBR, SBR, HNBR, XNBR, FVMQ, PMQ, PVMQ, MQ, VMQ, AU, and EU. In other exemplary embodiments, the O-ring 146 may exhibit certain properties after soaking in oil, for example, Mobil 254, for at least three hundred thirty-six (336) hours at 392° F. 110° F., such as having a compression set of less than about 20%, volume swell of less than about 12%, reduction in ultimate elongation damage of less than about 10%, and/or a reduction in tensile strength of less than about 10%.

Thus, there has now been provided a seal assembly that is less time-consuming to design and manufacture. Moreover, the seal assembly supplies an appropriate amount of friction against a seal ring, without become dislocated in the event of seal ring dislocation. In addition, the seal assembly minimizes leakage at the turbine section and output section of the ATS. The selection of the O-ring elastomer also uses a compression modulus of the elastomer and/or tensile modulus of the material, and/or shear (torsional or flexure/transverse) modulus of the material, instead of, or alternatively, in addition to, durometer hardness measurements for material selection in identifying which material may be used in low rolling friction applications such as mechanical face seals or solenoids or control valves.

While the invention has been described with reference to a preferred embodiment, it will be understood by those skilled in the art that various changes may be made and equivalents may be substituted for elements thereof without departing from the scope of the invention. In addition, many modifications may be made to adapt to a particular situation or material to the teachings of the invention without departing from the essential scope thereof. Therefore, it is intended that the invention not be limited to the particular embodiment disclosed as the best mode contemplated for carrying out this invention, but that the invention will include all embodiments falling within the scope of the appended claims. 

1. A stator seal assembly comprising: a seal case having an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween; an O-ring coupled to the seal case inner peripheral wall and disposed in the annular cavity, the O-ring comprising a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50; and a stator ring disposed within the seal case annular cavity.
 2. The seal of claim 1, wherein the material further comprises a fluorocarbon elastomer.
 3. The seal of claim 2, wherein the material further comprises at least one of perfluorocarbon, peroxide cured fluorocarbon, GLT fluorocarbon, and GFLT fluorocarbon.
 4. The seal of claim 1, wherein the material further comprises at least one of silicone, fluorosilicone, ethylene propylene, nitrile, butadiene, chloroprene, butyl, isoprene, fluoroelastomers, polyurethane, polyester, and, in accordance with ASTM D1418 abbreviations, the elastomer compounds of ACM, AEM, CSM, EPDM, EPM, FKM, FEPM, FFKM, CO, ECO, CO, ECO, BR, CR, IIR, CIIR, IR, NBR, SBR, HNBR, XNBR, FVMQ, PMQ, PVMQ, MQ, VMQ, AU, and EU.
 5. The seal of claim 1, wherein the stress versus strain ratio of the material is between about 0.1 and 5.0 ksi.
 6. The seal of claim 1, wherein, the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.+10° F. and having a compression set of less than about 20%.
 7. The seal of claim 6, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.+10° F. and having a volume swell of less than about 12%.
 8. The seal of claim 7, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.±10° F. and having a reduction in ultimate elongation damage of less than about 10%.
 9. The seal of claim 8, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.±10° F. and having a reduction in tensile strength of less than about 10%.
 10. A seal assembly comprising: a seal case having an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween; an O-ring coupled to the seal case inner peripheral wall and disposed in the annular cavity, the O-ring comprising a material having stress versus strain ratio of less than about 5.0 ksi and a Shore M value of about 50; and a stator ring disposed within the seal case annular cavity.
 11. The seal of claim 10, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.±10° F. and having a compression set of less than about 20%.
 12. The seal of claim 11, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.+10° F. and having a volume swell of less than about 12%.
 13. The seal of claim 12, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.±10° F. and having a reduction in ultimate elongation damage of less than about 10%.
 14. The seal of claim 13, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.±10° F. and having a reduction in tensile strength of less than about 10%.
 15. A face seal assembly comprising: a seal rotor adapted to mount to a shaft and having an axially facing flange; a seal case having an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween; an O-ring coupled to the seal case inner peripheral wall and disposed in the annular cavity, the O-ring comprising a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50; and a stator ring disposed within the seal case annular cavity and configured to engage the axially facing flange.
 16. The seal of claim 15, wherein the material further comprises a fluorocarbon elastomer.
 17. The seal of claim 16, wherein the material further comprises at least one of perfluorocarbon, peroxide cured fluorocarbon, GLT fluorocarbon, and GFLT fluorocarbon.
 18. The seal of claim 17, wherein the material further comprises at least one of silicone, fluorosilicone, ethylene propylene, nitrile, butadiene, chloroprene, butyl, isoprene, fluoroelastomers, polyurethane, polyester, and, in accordance with ASTM D1418 abbreviations, the elastomer compounds of ACM, AEM, CSM, EPDM, EPM, FKM, FEPM, FFKM, CO, ECO, CO, ECO, BR, CR, IIR, CIIR, IR, NBR, SBR, HNBR, XNBR, FVMQ, PMQ, PVMQ, MQ, VMQ, AU, and EU.
 19. The seal of claim 17, wherein the stress versus strain ratio is between about 0.1 and 5.0 ksi.
 20. The seal of claim 15, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.+10° F. and having a compression set of less than about 20%.
 21. The seal of claim 20, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.+10° F. and having a volume swell of less than about 12%.
 22. The seal of claim 21, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.+10° F. and having a reduction in ultimate elongation damage of less than about 10%.
 23. The seal of claim 22, wherein the material comprises material capable of being soaked in oil for at least three hundred thirty-six (336) hours at 392° F.±10° F. and having a reduction in tensile strength of less than about 10%.
 24. An air turbine starter comprising a rotatable shaft; a seal assembly coupled to the rotatable shaft, the seal assembly comprising: a seal rotor coupled to the rotatable shaft, the seal rotor having an axially facing flange; a seal case having an inner peripheral wall, an outer peripheral wall, and an annular cavity formed therebetween; an O-ring coupled to the seal case inner peripheral wall and disposed in the annular cavity, the O-ring comprising a material having stress versus strain ratio of less than about 5 ksi and a Shore M value of between about 30 and 50; and a stator ring disposed within the seal case annular cavity and configured to engage the axially facing flange.
 25. The seal of claim 24, wherein the material further comprises a fluorocarbon elastomer.
 26. The seal of claim 25, wherein the O-ring further comprises at least one of perfluorocarbon, peroxide cured fluorocarbon, GLT fluorocarbon, and GFLT fluorocarbon.
 27. The seal of claim 24, wherein the material further comprises at least one of silicone, fluorosilicone, ethylene propylene, nitrile, butadiene, chloroprene, butyl, isoprene, fluoroelastomers, polyurethane, polyester, and, in accordance with ASTM D1418 abbreviations, the elastomer compounds of ACM, AEM, CSM, EPDM, EPM, FKM, FEPM, FFKM, CO, ECO, CO, ECO, BR, CR, IIR, CIIR, IR, NBR, SBR, HNBR, XNBR, FVMQ, PMQ, PVMQ, MQ, VMQ, AU, and EU.
 28. The seal of claim 24, wherein the stress versus strain ratio is between about 0.1 and 5.0 ksi. 